The invention relates to a method for predicting values of harmonic distortion in an AC power system and a method for applying power-factor correction in such a system.
In many AC power systems, usually 3-phase, the loading placed on such systems is very often highly inductive in nature, typically resulting from the operation of heavy electrical machines having a pronounced lagging power factor. In such a situation it is known to attempt to correct for this by the imposition of a known value of capacitance across the AC busbars. However, while it may be an easy matter to apply an appropriate value of capacitance to the system under static loading conditions, in practice AC systems tend to have very unpredictable loading, such that a value of capacitance which may be correct at any one time may be entirely incorrect at a later time.
To cater for such changing conditions, banks of parallel-connected capacitors are often provided, the number of banks which are applied to the system being selected in response to the changing loading conditions. It is a known problem, however, that under certain conditions of capacitance loading a resonance state can be set up between the applied capacitance and the inductance of the power system. Such resonances can have the effect of greatly magnifying the existing harmonic content of the supply voltage and current, thereby causing considerable corruption to the supply waveform. In order to avoid this, it is a commonly used technique to install either harmonic-filtration or harmonic-blocking reactors to the system.
The main problem with these methods is that the installation of the reactive power compensation will typically double in price compared to the use of a simple capacitor bank and will also give rise to a very lossy system, both of which drawbacks weaken the argument for installing such equipment.